In prior U.S. Pat. No. 3,443,060, a plastic bag carrying blood for a patient was maintained at a thermostatically controlled temperature by a cylindrical heated mandrel around which a flat plastic bag was telescoped, and an outer hollow cylindrical mandrel was telescoped over the bag on the inner mandrel. Longitudinal ribs formed on one of the mandrels pressed the bag walls together under the ribs and restricted the bag to form a labyrinth of passageways through the bag, with inlet and outlet connections for conducting the blood supply to the bag and from the bag to the patent.
Properly inserting and fitting the bag between the inner and outer mandrels in the construction of prior patent No. 3,443,060 is a difficult and time-consuming operation, and the particular shape of the bag is costly to fabricate.